A defibrillator is a device used to administer a high intensity electrical shock through a pair of electrodes, or "paddles," to the chest of a patient in cardiac arrest. A selected, discrete quantity of energy is typically stored in a capacitor and is then electrically discharged into the patient through the paddle circuit.
Defibrillation is not a procedure with a certain and successful outcome. Rather, the probability of successful defibrillation depends on the condition of the patient and on the defibrillation discharge parameters. In order to practice defibrillation successfully and safely, it is important to quickly make an optimum choice of the defibrillation discharge intensity level. If the selected discharge intensity level is too low, defibrillation will not be successful and must be repeated at a higher intensity level until the patient is defibrillated. However, repeated defibrillation discharges at increasing intensity levels are more likely to cause damage to the heart. Also repeated discharges cause the patient to remain in ventricular fibrillation for a longer time. This causes the patient's condition to deteriorate, as metabolic imbalance and hypoxia develop, which, in turn, make the patient harder to defibrillate and reduce the prospect of successful recovery. One must, therefore, attempt to defibrillate as quickly as possible, and one must not initially attempt defibrillation at too low a discharge intensity.
On the other hand, it is well known that the probability of successful defibrillation increases, maximizes, and then decreases as the discharge intensity is steadily increased. (c.f. J. C. Schuder et al., "Transthoracic Ventricular Defibrillation," IEEE Transactions on Biomedical Engineering, Vol. BME-27, pp. 37-43, 1980.) Hence, one must also avoid excessive intensity levels. Excessive intensity levels not only reduce the probability of successful defibrillation, but also increase the risk of damaging the heart (myocardium and nerve system) as a result of an excessive discharge current flowing through the heart.
Due to these countervailing considerations, there have been a number of attempts to determine the effect of various discharge parameters on the efficacy and safety of defibrillation in order to help defibrillation operators choose the optimum intensity level for defibrillation.
(It should be noted that the general term "intensity" has been purposely used here to describe the level of the discharge. Intensity may be measured by any one of several discharge parameters such as energy stored in the defibrillator, energy delivered to the patient, peak or average current flowing through the patient, electrical charge or integrated electron flow through the patient, etc.)
In the following discussion, the defibrillation parameters which have been commonly used in the past are reviewed, with their corresponding limitations and inadequacies. The next discussion then identifies a new parameter which has not been used or discussed in the literature, but which appears to be the most general and accurate parameter which the operator can select and control in order to optimize the probability of successful and safe defibrillation on the first attempt.
Traditionally, defibrillators have been designed to allow control and selection of the energy E.sub.s stored in the defibrillator capacitor. This is equivalent to selection of the energy E.sub.d (50) which will be delivered into a 50 ohm load, i.e., delivered into the patient if a patient transthoracic impedance (Z.sub.p) of 50 ohms is assumed. In fact, all defibrillator currently on the market are designed to permit the operator to select E.sub.d (50) only. Partly for that reason, generally accepted protocols for defibrillation, such as that published by the American Heart Association in the Journal of the American Medical Association ("Standards and Guidelines for Cardiopulmonary Resuscitation and Energy Cardiac Care," JAMA, Vol. 255, No. 21 pp. 2841-3044, Jun. 6, 1986) recommend that defibrillation be performed at an E.sub.d (50) of 200 Joules for the first two attempts (for adults in ventricular fibrillation) then at 300 or 360 Joules for subsequent attempts if required.
Unfortunately, the ability to select E.sub.d (50) does not optimize the probability of safe and successful defibrillation. This is due in large part to the fact that the patient's Z.sub.p is generally rot known prior to discharge, and it is well known that Z.sub.p varies widely from patient to patient (from 15 to 140 ohms, with a mean of approximately 65 ohms, as reported in Kerber et al., "Transthoracic Resistance in Human Defibrillation, Influence of Body Weight, Chest Size, Serial Shocks, Paddle Size and Paddle Contact Pressure," Circulation, Vol. 63, No. 3, March 1981). The energy actually delivered to the patient depends on the value of Z.sub.p and will be markedly different from the intended energy E.sub.d (50) if the patient Z.sub.p is either much lower or much higher than 50 ohms.
Delivered energy is not a satisfactory parameter to control defibrillator success and safety, since both the energy required for defibrillation success and the energy threshold for damage to the heart depend strongly on the patient's Z.sub.p. That is, a patient with a very low Z.sub.p (e.g., 25 ohms) may be successfully defibrillated by an energy of only 50 Joules and may suffer heart damage from an energy as low as 200 Joules, whereas a patient with a high Z.sub.p (e.g., 100 ohms) may require 300 Joules for successful defibrillation and may suffer damage only at much higher energies, e.g., 500 Joules or more.
Since the drawbacks of the selected energy approach have been recognized, there have been searches for better control parameters.
Generally accepted studies indicate that the basic processes of defibrillation and myocardial damage are more closely related to the flow of electrons through the heart than to the energy of the discharge. For example, Dr. Kerber in 1984 investigated the threshold for successful defibrillation in a patient population divided into two groups according to their Z.sub.p. (Kerber et al., "Advance Prediction of Transthoracic Impedance in Human Defibrillation and Cardioversion: Importance of Impedance in Determining the Success of Low-Energy Shocks," Vol. 70, No. 2, August 1984.) The low-to-average Z.sub.p group had an average defibrillation threshold of 135 Joules (delivered energy) and 29 amperes (peak current), whereas the high Z.sub.p group had an average threshold of 211 Joules and 28 amperes. Hence, the defibrillation threshold measured by the peak current was the same for all patients irrespective of Z.sub.p, whereas the defibrillation threshold measured by the delivered energy did increase with Z.sub.p. These findings were supported by two later studies (Kerber et al., "Energy, Current and Success in Defibrillation and Cardioversion: Clinical Studies Using an Automated Impedance-Based Method of Energy Adjustment," Circulation, Vol. 77, No. 5, May 1988; and Lerman et al., "Current-Based Versus Energy-based Ventricular Defibrillation: A Prospective Study," American College of Cardiology, Vol. 12, No. 5, pp. 1259-64, November 1988). These studies led to a proposal that peak current I.sub.m be used, rather than energy E.sub.d, as the control parameter selected for defibrillation.
Although peak current I.sub.m is a better choice than energy E.sub.d, it still suffers from serious limitations, since the appropriate value of I.sub.m which yields a high probability of defibrillation success (while minimizing the risk of damage to the heart) depends strongly on the defibrillator discharge waveform.
For instance, in a study of defibrillation using trapezoidal waveforms of low or high tilt, Bourland showed that, for a given pulse duration d, high tilt waveforms required a much higher peak current I.sub.m than low tilt waveforms for successful defibrillation, while the average current I.sub.m was essentially the same for all values of tilt from 0 to 90%. (Bourland et al., "Strength-Duration Curves for Trapezoidal Waveforms of Various Tilts for Transchest Defibrillation in Animals," Medical lnstrumentation, Vol. 12, No. 1, pp. 38-41, 1978.) Bourland then extended his study to another class of defibrillation discharge waveforms, i.e., the damped sinusoidal waveforms (DSW) which are used in almost all defibrillators currently on the market. Again, he found that the peak current varied markedly but, for a given duration of discharge, the average current I.sub.a required for successful defibrillation was essentially the same for all waveforms considered, i.e., DSW and trapezoidal with varying tilt. (Bourland et al., "Comparative Efficacy of Damped Sine Wave and Square Wave Current for Trans-chest Ventricular Defibrillation in Animals," Medical Instrumentation, vol. 12, no. 1, pp 42-45, 1978.) For a given pulse duration d, it appears that the peak current I.sub.m required for successful defibrillation varies widely with waveform, but the average current I.sub.a or, more accurately, the integrated electron flow or charge, is a more general parameter controlling the success of defibrillation.
However, one more generalization is needed to identify a parameter with universal validity, i.e., a parameter fairly independent of pulse duration, since it is known that various defibrillators currently on the market use circuit components (capacitors and inductors) which result in different discharge durations (generally in the 2 to 8 millisecond range for damped sinusoidal waveforms, but up to 15 or 20 milliseconds for certain trapezoidal waveform defibrillations).
It is well known that the discharge parameters (peak or average current, charge, energy) required for successful defibrillation vary with pulse duration. The laws of tissue stimulation were characterized at the turn of this century and can be expressed by the following equations: ##EQU1## where d is the pulse duration, and t and k are constants characteristic of tissue. These relationships are represented graphically by the familiar strength-duration curves (Geddes et al., "Fundamental Criteria Underlying the Efficacy and Safety of Defibrillating Current Waveforms," Medical and Biological Engineering and Computing, Vol. 23, pp. 122-130, 1985).
A number of studies of internal and external defibrillation using a wide variety of waveforms have shown that the defibrillation threshold parameters follow the above-noted relationships fairly well, with b and k such that the minimum energy defibrillation occurs at a pulse duration of 3 to 5 milliseconds. Over the range of pulse duration most commonly (and justifiably) used for clinical defibrillation, i.e., 2 to 8 milliseconds, average current I.sub.a decreases as d increases, Q increases with d, but the product I.sub.a .multidot.Q, i.e., the ratio of discharge energy to patient transthoracic impedance, is essentially constant.
The foregoing discussion serves as introduction to our conclusion that there is a parameter which can be used as a predictor of successful defibrillation, so that a certain chosen value of this parameter can be associated with a given desired probability of successful defibrillation. This parameter value is universal in that it is essentially independent of patient transthoracic impedance and of discharge pulse waveform morphology and duration (whereas the optimum value of I.sub.a or Q depend on pulse duration, that of I.sub.m depends on both waveform morphology and pulse duration, and that of delivered energy depends on patient Z.sub.p). This universal parameter is the impedance-corrected delivered energy E.sub.d /Z.sub.p in Joules per ohm.
Using data collected in a recent experimental study of human defibrillation (Kerber et al., Circulation, Vol. 77, No. 5, May 1988, supra) and in a subsequent ongoing multicenter study coordinated by Kerber and not yet published, it can be confirmed that the probability of successful defibrillation is closely correlated to E.sub.d /Z.sub.p for the various malignant arrhythmias (ventricular fibrillation, atrial fibrillation, atrial flutter, monomorphic or polymorphic ventricular tachycardia) which are commonly treated by defibrillation or by synchronized cardioversion. In the most important case, i.e., that of ventricular fibrillation, FIG. 5 shows the observed defibrillation success rate as a function of E.sub.d /Z.sub.p for the current multicenter study. A minimum value E.sub.d /Z.sub.p of approximately 3 Joules per ohm is required to achieve a high probability (70%) of successful defibrillation. The highest success probability (87%) is achieved for values of E.sub.d /Z.sub.p in the range of 3.75 to 4.50.
The standard (energy based) defibrillation protocol published by the American Heart Association and the American Medical Association recommends the following sequence:
First attempt at 200 Joules; PA0 Second attempt at 200 Joules; and PA0 If unsuccessful, increase the energy to 300 or 360 Joules for subsequent attempts. PA0 First defibrillation attempt at 3 Joules per ohm; PA0 Second defibrillation attempt at 3 Joules per ohm; and PA0 If unsuccessful, increase the defibrillator output to 4.5 Joules per ohm for subsequent attempts.
For reasons discussed earlier, this protocol has limited efficacy because patient Z.sub.p is highly variable and recommended initial level of 200 Joules is too low for patients with high Z.sub.p (e.g., 100 ohms) and is excessive (i.e., unnecessary risk of heart damage) for patients with low Z.sub.p (e.g., 25 ohms). According to the present invention, a defibrillation protocol based on the parameter E.sub.d /Z.sub.p is proposed. An exemplary methodology is as follows:
An alternate, more aggressive protocol would be to select a E.sub.d /Z.sub.p value of 4 for initial defibrillation
The foregoing and additional advantages of the present invention will be more readily apparent from the owing detailed description thereof, which proceeds with reference to the accompanying drawings.